1. Field of the Invention
The present invention relates to a document reading apparatus that reads a document image and an imaging apparatus such as a scanner, a digital copier, or a facsimile machine that implements this document reading apparatus.
2. Description of the Related Art
In the conventional document reading apparatus, inconsistencies in the density representation values of output signals from a linear image sensor representing the same density may be due to, for example, an inconsistency in the initial light level of the light source, a short-term fluctuation of the light level, a long-term degradation of the light level (degradation over time), or an unevenness in sensor sensitivity, in the case of black-and-white image reading. Also, in the case of color image reading, inconsistencies in the output signals representing image density may be due to an inconsistency in the initial color level of the light source, a short-term fluctuation in the color level of the light source, or a long-term fluctuation of the color level of the light source, for example. Further, due to these inconsistencies and fluctuations, the maximum output may be approximately 6˜7 times the minimum output.
Thus, in the conventional art, the above-described inconsistencies are compensated by performing variable gain amplification in an analog signal processing step implemented after the linear image sensor process. However, in the conventional technique, it is difficult to secure a stable density performance in such a wide dynamic range of the image signals, and also, a large load is imposed on the analog signal processing circuit thereby hindering the simplification of the analog signal processing unit.
In Japanese Patent Laid-Open Publication No.10-285338, a technique is disclosed in which the start up time of a document irradiating fluorescent lamp of a scanner is shortened, and after the start up time, the fluorescent lamp is driven with high frequency in order to stabilize the light level. Further, a PWM (pulse width modulation) control is performed upon driving the fluorescent lamp and the amount of light irradiated by the fluorescent lamp is detected with a photodiode. This detection result is compared with a light level standard signal and if it is determined that the amount of light is insufficient, the PWM duty is increased. On the other hand, if the light level is determined to be excessive, the duty is decreased. This up-down fluctuation of the duty is greater during start up than after the start up. The pulse period of the PWM pulse corresponds to the period of a line synchronization signal SYNC. The fluorescent lamp is driven by a high frequency during the earlier half of the accumulation time for a CCD to accumulate signals of one line. The high frequency lighting is turned off during the latter half of the accumulation time.
In Japanese Patent Laid-Open Publication No.2001-273996, an apparatus is disclosed in which two discharge lamps each having an inverter for providing a stable irradiation of high luminance light are interchangeably used.
According to the conventional art, when a fluorescent lamp is turned on and off through PWM control of an inverter lighting the lamp, a clock pulse from a counter determining the on pulse width of the PWM pulse and the high frequency high voltage applied to the fluorescent lamp by the inverter are not synchronized, and thereby, even if the duty of the PWM is the same, the amount of light irradiated by the fluorescent lamp within one period is uneven. That is, the amount of exposure is varied for each line.